The present invention relates generally to crystal growing apparatus used in growing monocrystalline ingots, and more particularly to an electrical resistance heater for use in such a crystal growing apparatus.
Single crystal silicon, which is the starting material for most semiconductor electronic component fabrication, is commonly prepared by the so-called Czochralski (xe2x80x9cCzxe2x80x9d) method. The growth of the crystal is most commonly carried out in a crystal pulling furnace. In this method, polycrystalline silicon (xe2x80x9cpolysiliconxe2x80x9d) is charged to a crucible and melted by a heater surrounding the outer surface of the crucible side wall. A seed crystal is brought into contact with the molten silicon and a single crystal ingot is grown by slow extraction via a crystal puller. After formation of a neck is complete, the diameter of the crystal ingot is enlarged by decreasing the pulling rate and/or the melt temperature until the desired or target diameter is reached. The cylindrical main body of the crystal which has an approximately constant diameter is then grown by controlling the pull rate and the melt temperature while compensating for the decreasing melt level. Near the end of the growth process, the crystal diameter must be reduced gradually to form an end-cone. Typically, the end-cone is formed by increasing the pull rate and heat supplied to the crucible. When the diameter becomes small enough, the ingot is then separated from the melt.
Heaters used for melting source material (e.g. silicon) in the crucible are typically electrical resistance heaters in which an electrical current flows through a heating element constructed of a resistive heating material (e.g., graphite). The resistance to the flow of current generates heat that radiates from the heating element to the crucible and silicon contained therein. The heating element comprises vertically oriented heating segments of equal length and cross-section arranged in side-by-side relationship and connected to each other in a serpentine configuration. That is, adjacent segments are connected to each other at the tops or bottoms of the segments in an alternating manner to form a continuous electrical circuit throughout the heating element. The heating power generated by the heating element is generally a function of the cross-sectional area of the segments.
Although the conventional apparatus used for growing single crystal ingots according to the Czochralski growth method has been satisfactory for growing crystal ingots useful in a wide variety of applications, further improvements in the quality of the semiconductor material are desirable. As the width of integrated circuit lines formed on the semiconductor material continue to be reduced, the presence of defects in the crystal become of greater concern. A number of defects in single crystal silicon form in the crystal growth chamber as the crystal cools after solidification. Such defects arise, in part, because of the presence of an excess (i.e., a concentration above the solubility limit) of intrinsic point defects, which are known as vacancies and self-interstitials. It has been suggested that the type and initial concentration of these point defects in the crystal ingot are determined at the time of solidification and, if these concentrations reach a level of critical supersaturation in the system and mobility of point defects is sufficiently high, a reaction, or an agglomeration event, will likely occur.
One important measurement of the quality of wafers sliced from a single-crystal ingot is Gate Oxide Integrity (xe2x80x9cGOIxe2x80x9d). Vacancies, as their name suggests, are caused by the absence or xe2x80x9cvacancyxe2x80x9d of a silicon atom in the crystal lattice. When the crystal is pulled upward from the molten silicon in the crucible, it immediately begins to cool. As the temperature of the crystal ingot descends through the temperature range of 1150xc2x0 C. down to 1050xc2x0 C., vacancies present in the ingot tend to migrate out toward the outer surface of the ingot or agglomerate together within the ingot. These agglomerations are manifested as pits within the surfaces of the wafers sliced from the crystal ingot.
Silicon wafers sliced from the ingot and manufactured according to conventional processes often include a silicon oxide layer formed on the surface of the wafer. Electronic circuit devices such as MOS devices are fabricated on this silicon oxide layer. Defects in the surface of the wafer, caused by the agglomerations present in the growing crystal, lead to poor growth of the oxide layer. The quality of the oxide layer, often referred to as the oxide film dielectric breakdown strength, may be quantitatively measured by fabricating MOS devices on the oxide layer and testing the devices. The Gate Oxide Integrity (GOI) of the crystal is the percentage of operational devices on the oxide layer of the wafers processed from the crystal.
It has been determined that the GOI of crystals grown by the Czochralski method can be improved by increasing the amount of time a growing ingot dwells in the temperature range above 1000xc2x0 C., and more particularly in the range of 1150xc2x0 C.-1000xc2x0 C. If the ingot cools too quickly through this temperature range, the vacancies will not have sufficient time to agglomerate together, resulting in a large number of small agglomerations within the ingot. This undesirably leads to a large number of small pits spread over the surfaces of the wafer, thereby negatively affecting GOI. Slowing down the cooling rate of the ingot so that its temperature dwells in the target temperature range for a longer period of time allows more vacancies to move to the outer surface of the ingot or form large agglomerations within the ingot. The result is a small number of large agglomerations, thereby improving GOI by decreasing the number of defects present in the surface of the wafer upon which the MOS devices are formed.
Defects relating to self-interstitials are less well studied. They are generally regarded as being low densities of interstitial-type dislocation loops or networks. Such defects are not responsible for GOI failures, but they are widely recognized as the cause of other types of device failures usually associated with current leakage problems. It has been determined that the agglomeration of self-interstitials are undesirable and can be controlled by increasing the amount of time a growing ingot dwells in the temperature range above 1000xc2x0 C. As portions of the ingot remain at temperatures above 1000xc2x0 C. for relatively long time durations, radial out-diffusion of self-interstitials from the ingot occurs to suppress the concentration below the critical concentration required for agglomeration of interstitial defects.
To these ends, U.S. Pat. No. 5,248,378 (Oda et al.) discloses an apparatus for producing single silicon crystal in which a passive heat insulator is disposed in the crystal puller above the crucible to reduce the rate of cooling of the growing ingot above 1150xc2x0 C. However, heat insulators or heat shields such as that disclosed by Oda et al. generally cannot slow the cooling of the ingot to a rate sufficient to substantially improve the GOI of the crystal or suppress the agglomeration of interstitial defects.
Oda et al. further disclose that the insulator may be replaced by a heater for heating the growing ingot. While a heater similar to the conventional crucible heater described above would more actively apply heat to the ingot to reduce the cooling rate, using such a heater has a number of disadvantages. For example, the heating power output of the conventional heater is generally constant along the height of the heater. A crystal ingot being pulled upward through the heater would be rapidly heated at the bottom of the heater to reduce the cooling rate of the ingot. The cooling rate would continue to decrease as the ingot passed upward through the heater and then increase upon reaching the top of the heater. For example, a plot of the axial temperature gradient of the ingot versus the ingot temperature would look similar to that shown in FIG. 5 for an ingot grown without a secondary heater, but the axial temperature gradient would be substantially reduced throughout the temperature range of 1150xc2x0 C.-1000xc2x0 C. including the valley. Thus, such a heater is capable of reducing the cooling rate, or axial temperature gradient, of the growing ingot, but it cannot control the uniformity of the axial temperature gradient because the heat generated by the heater is uniform along the length of the heater. By significantly reducing the cooling rate of the heater to a defined valley, it will take an undesirably long time for the cooling rate to increase once the ingot cools to a temperature of 1000xc2x0 C.
There is a need, therefore, for a heater that can be used to both reduce the cooling rate of the growing ingot above 1000xc2x0 C. and control the uniformity of the axial temperature gradient of the ingot in the range of 1150xc2x0 C.-1000xc2x0 C., and more particularly in the range of 1100xc2x0 C.-1000xc2x0 C.
Among the several objects and features of the present invention may be noted the provision of an electrical resistance heater for use in a crystal puller which facilitates the growth of crystals having an improved Gate Oxide Integrity; the provision of such a heater which provides a more uniform axial temperature gradient in the range of 1100xc2x0 C.-1000xc2x0 C.; the provision of such a heater which increases the dwell time of the ingot at a temperature above 1000xc2x0 C.; the provision of such a heater which increases the dwell time of the ingot temperature in the range of 1100xc2x0 C. to 1000xc2x0 C.; and the provision of such a heater which facilitates reduction of agglomeration of intrinsic point defects.
Generally, an electrical resistance heater of the present invention for use in a crystal puller used for growing monocrystalline silicon ingots according to the Czochralski method comprises a heating element sized and shaped for placement in the housing of the crystal puller generally above the crucible in spaced relationship with the outer surface of the growing ingot for radiating heat to the ingot as it is pulled upward in the housing relative to the molten silicon. The heating element has an upper end and a lower end. The lower end of the heating element is disposed substantially closer to the molten silicon than the upper end when the heating element is placed in the housing. The heating element is constructed such that the heating power output generated by the heating element gradually increases from the lower end to the upper end of the heating element.
In another embodiment, a method of controlling the uniformity of the axial temperature gradient of a monocrystalline silicon ingot grown in a crystal puller used for growing monocrystalline silicon ingots according to the Czochralski method comprises allowing the ingot to cool to a predetermined first temperature as it is pulled upward in the housing from the molten silicon. Heat is radiated to the ingot as it is pulled further upward in the housing to reduce the cooling rate of the ingot as it cools down from the first temperature. The amount of heat radiated to the ingot is gradually increased as the ingot is pulled further upward in the housing to maintain a generally uniform axial temperature gradient of the ingot as the temperature of the ingot decreases from the first temperature down to a predetermined second temperature.
Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter.